The invention relates generally to generally to devices for separating electrons or other charged particles according to their energies. Specifically, the present invention relates to real-time analysis of the low-energy portion of the energy distribution of such particles.
Kelly in U.S. Pat. Nos. 5,583,336 and 5,969,354 describes an electron analyzer for the real-time detection of the entire energy distribution of a beam of electrons. Both these patents are incorporated herein by reference in the their entireties. The analyzer of the ""336 patent is illustrated in FIG. 1. Its central part is an energy separator 10 that uses a solenoid 12 powered by an unillustrated DC current supply to create a substantially uniform magnetic field B in a separation region 14. The magnetic field is terminated by magnetic meshes 16, 18 linked by cylindrical magnetic yoke 20. The ""354 patent generalizes this configuration to non-uniform magnetic fields and magnetic meshes acting as lenses.
A collimated electron source beam 24 is offset by a polar angle xcex8 from a longitudinal axis 26 of the energy separator 12. It is assumed that the azimuthal angle xcfx86 lies within the plane of the illustration, which corresponds to a value of zero. The source beam 24 originates from an electron source 28 and is collimated by a set of beam optics 30.
It is well known that a charged particle entering a magnetic field region at an oblique angle to the magnetic field, here parallel to the central axis 26, will gyrate about the local magnetic field line B in a helical path 32 so as to maintain its polar angle xcex8 but to have its azimuthal angle xcfx86 with respect to the local magnetic field line B increasing linearly with time or distance with the proportionality constant dependent upon magnetic field and the particle""s kinetic energy and charge. These two angles xcex8 and xcfx86 determine the instant velocity of the charged particle with respect to the central axis 26. Assuming the same initial polar and azimuthal angles xcex80 and xcfx860 for all energies, the total angular rotation xcex94xcfx86 will depend upon the particle""s energy. As a result, electrons of differing energies and all entering the separator 10 with the same polar angle xcex80 and the same azimuthal angle xcfx860 will exit the separator on a path at an angle dependent upon the electron energy. In fact, the polar angles will all be the same xcex8 but the azimuthal angles xcfx86 will be energy dependent. If only a limited energy range is being analyzed, a distribution of the electrons as a function of the exit azimuthal angle xcfx86 will give the energy distribution. However, for larger energy ranges it is necessary to resolve the uncertainty in multiples of 2xcfx80 in the azimuthal angle. This is accomplished with electrostatic biasing between the exit magnetic grid 18 and an electrical grid 34 which provides an energy dependent change of the polar angle xcex8.
Beam optics 36 focus the electrons on an imaging screen 38 or other radiation detector arranged in two dimensions, for example, a 2-dimensional CCD imaging array. As illustrated in FIG. 2, the electrons will fall along a spiral locus 40 with its two ends 42, 44 defining the two limits of the detected energy range and the portions in between having a one-to-one correspondence to intermediate energies. The distribution of intensities along the spiral locus 40 corresponds to the energy distribution of the electrons.
One intended use of the energy analyzer of FIG. 1 is as the analyzer for an Auger electron spectrometer. Auger spectrometers are commercially available from several sources, including Physical Electronics (PHI), a division of Perkin-Elmer of Eden Prairie, Minnesota, Vacuum Generators of the United Kingdom, and Omicron of Delaware.
In the typical practice of Auger spectroscopy, the solid is probed with a beam of primary electrons in the low keV range of energies and produces a secondary electron through an Auger transition process having a well defined Auger energy EAUGER. In Auger spectroscopy, the probing radiation ejects an inner-shell electron from an atom. Then in the Auger transition, a first outer-shell electron falls into the inner-shell vacancy and a second outer-shell electron is ejected carrying the difference in energy. The spectrometer analyzes the energy of the ejected electron as the Auger energy EAUGER. Auger energies are generally in the range of a few hundred eV to a few keV for the typical practice of Auger electron spectroscopy. The Auger energy EAUGER is for the most part unique for each atom, primarily dependent upon the atomic number Z but also depends on the bonding with neighboring atoms. Thus, the measured electron energy can be used to determine the composition of the material, at least near its surface. Auger electron spectroscopy allows the very quick and highly accurate measurement of film thicknesses up to about 20 nm.
Auger energies are typically in the range of a few hundreds to a few thousands of electron volts (eV). Because of the multiple electron transitions, the Auger energy is typically less than half of the primary energy Ep. Further, to enhance the Auger signal, the primary energy Ep is increased even more, often to five times the Auger energy.
Auger signals are generally relatively weak compared to the primary beam. Also, other processes can occur as the primary beam propagates into the material. As a result, the Auger signal is often difficult to extract from a greater number of electrons at the same energy. In addition, electrons of higher energy leave the surface, and these electrons can degrade the, analysis system. Nonetheless, the electron analyzer of FIGS. 1 and 2 must be designed to accurately treat all energies of electrons incident upon it even though when used in an Auger spectrometer only the lower energies are of interest.
Accordingly, it is desired to provide an electron spectrometer that can more effectively and accurately determine the energies of the lower-energy portion of an energy distribution. It is further desired to integrate such means with the energy analyzer of FIG. 1 with the addition of only a few elements.
The invention includes a low-pass energy filter on the input to an energy analyzer for charged particles.
The invention includes an electron energy analyzer including a reflector positioned at the input acting as a low-pass filter. The reflector includes a grid held at a predetermined potential, preferably grounded, and an electrode disposed in back of it held at a more positive potential and configured to reflect electrons with a lower potential energy than the negative electrode potential. These electrons with less energy are reflected and accelerated back through the front grid electrode.
The plane of the reflector is inclined to the analyzer axis so that low-energy electrons reflected from the low-pass filter enter the analyzer at an oblique angle. The analyzer is preferably composed of a drift region having an axial magnetic field around which the energies gyrate in a spiral pattern. The amount of rotation accumulates along the drift region and depends upon the electron energy.
In one embodiment, the reflector is planar. A collimated beam striking the reflector is collimated upon entering the drift region.
In a second embodiment, the reflector is also planar but receives a beam from a light source having its energy components being angularly dispersed so that the components enter the drift region with an energy-dependent oblique angle.
In a third embodiment, the reflector is curved, preferably parabolically or as a paraboloid so as to both energy filter and collimate the beam entering the drift region.
In a fourth embodiment, a second electron reflector is positioned at the end of the drift region opposite the input end, and the electrons are extracted from the input side of the drift region.